The invention concerns a beam splitter to divide an electromagnetic beam into at least two partial beams. In particular, the invention refers to such a beam splitter with essentially flat functional elements that divide the beam.
There already exist a series of various technologies for the above-cited beam splitter to cause the cited division of the beam. There operation is based on optical properties of a basic dielectric, and especially reflection of the incident beam from optical boundary surfaces is used. Such a beam splitter is shown in FIG. 1a. An incident beam 10 falls on the front outer surface 11 of a dielectric 12 and is partially reflected there 13. The transmitted beam part 14 then penetrates 15 a rear outer surface 16 of the dielectric 12 and is also partially reflected there 17. Some of the partially reflected beam 17 then leaves 18 the front outer surface 11 of the dielectric 12 and forms a partial beam bundle 13, 18 together with partial beam 13. Note that FIG. 1a is greatly simplified; when partial beam 17 contacts the front outer surface 11, additional reflection arises what is not represented here.
In these simplified representations, we can see two basic problems with the cited beam splitter. On one hand, so-called ghost images arise due to the displayed double reflection which substantially impair the beam properties of the reflected beam and, in particular, cause beam expansion. In addition, the arising parallel displacement of the penetrating (transmitted) beam produces axial image displacement, which is also frequently undesirable or even unacceptable.
The cited problems have already led to the development of beam splitters in which the thickness of the dielectric (viewed in the direction of the transmitted beam) is extremely thin to minimize the cited effects arising from multiple reflections and parallel displacement of the penetrating beam. Such a membrane beam splitter is schematically illustrated in FIG. 1b. This beam splitter has a membrane 21 in a frame 20. An incident beam is partially transmitted 23 through the membrane as well as partially reflected 24. These membrane beam splitters 20,21 are designed to be thin enough (approximately two pm nitrocellulose film) so that both reflected beam bundles 24 are practically completely superposed, i.e., there is no beam displacement or beam expansion (as described above). This beam splitter has the disadvantage, however, that is not suitable for the outside UV range; in addition, the interference that usually arises with thin membranes is greatly disturbing when such beam splitters are used.
Furthermore, other beam splitters have been suggested in which the outer surfaces are wedge-shaped to block the reflected partial beams; however, the bend in the optical axis is disadvantageous.
The above-described relevant state-of-the-art is also published in a catalog by Oriel Instruments under the title of The Book of Photon Tools in the section xe2x80x9cPrism and Beam Splitters, Beam Splitters Technical Discussionxe2x80x9d. FIGS. 1 and 2 are taken from this catalog. The cited disadvantages have also produced a completely different approach which is also described in the catalog. The basic beam splitter principle will now be briefly discussed with reference to the perspective view in FIG. 2 (with an enlarged section shown in the right half of the figure) of such a beam splitter. With this beam splitter, the beam is divided by means of a point pattern 31 on a highly-reflective material on a dielectric 30; an aluminum layer in this case. With this beam splitter, part of an incident beam is reflected off the point pattern 31, and the other part is transmitted through the uncoated areas 32 of the dielectric 30. By tilting the beam splitter in reference to the optical axis of the incident beam, the incident beam can be divided into partial beams similar to the method shown in FIG. 1.
All of the above described beam splitter technologies still have substantial disadvantages. On one hand, chromatic, spherical and astigmatic image errors arise, and there is also a relatively large reflection loss and intensity loss as the beam passes through the dielectric. As the beam passes through the dielectric, and initially unpolarized beam becomes partially polarized due to the angle of installation required to divide the beam. The cited disadvantages can at least be reduced by additional measures, however. For example, the intensity of the cited ghost images can be reduced e.g. by using reflection-reducing coatings and/or even sup pressed by blocking out disturbing reflections.
It is therefore the problem of the present invention to describe and present a beam splitter of the above-described kind that avoids the cited disadvantages of state-of-the-art beam splitters.
In particular, optical image errors such as the generation of ghost images, parallel displacement between incident and exiting beams as well as axial beam displacement or image haziness are effectively avoided as much as possible.
In addition, the reflection and transmission loss in the cited dielectrics is avoided as much as possible.
Furthermore, the invention prevents in particular the reflected beams from becoming polarized at certain angles of incidence due to reflection, and therefore prevents the exiting partial beams from being polarized in contrast to the incident beam.
In addition, the beam splitter is useful over the entire optical wavelength range and is highly suitable for the ultraviolet range of the spectrum.
Finally, the beam splitter is easier and hence more economical to manufacture than prior-art beam splitters.
The cited problems are solved with a beam splitter of the initially-cited type in that the functional element has openings or perforations corresponding to a point pattern. The areas of the functional element between the openings are essentially reflective, at least on the side the incident beam, and are essentially designed to form parallel exiting partial beams.
The incident beam is divided by the beam splitter according to the invention as follows: a portion of the incident beam (the portion contacting the membrane at the height of the openings) can freely pass through the beam splitter, and the remaining portion of the incident beam is reflected off the areas of the functional element between the openings. Since the transmitted beam portion is transmitted exclusively in air, an optical active medium such as a dielectric can accordingly be dispensed with; there is therefore no intensity loss, polarization, or imaging errors of the transmitted beam as is the case with state-of-the-art beam splitters as described in the introduction. By suitably selecting a reflective coating on the side of the functional element facing the incident beam, the reflected beam portion can also exit the beam splitter almost without any loss in intensity and without any polarization.
The functional element is preferably designed as a membrane that can be manufactured from a material that is essentially non-transparent to the respective electromagnetic beam or a material that is impermeable to the beam, in particular from a semiconductor material, metal, etc. The membrane with the corresponding openings can be manufactured by micromechanical means in particular, which allows the openings and especially their opening cross-sections to be microscopic so that the beam splitter can be used for a wide range of beam diameters. This advantageously allows the beam splitter to be used in the field of laser optics. For the cross-section of the incident beam to cover a sufficient number of openings to ensure a fixed and predetermined divider ratio, the average distance between two openings can be smaller by at least a factor of 2 to 5 than the diameter of the incident beam.
The functional element is preferably approximately 1-100 xcexcm thick at the openings. The respective micromechanical manufacturing procedure depends upon the respective starting material, i.e., photolithographic etching techniques or laser-support etching of semiconductor materials, or mechanical or chemical thinning of a metal sheet or film followed by drilling (especially micro-drilling) the metal sheet or film created in this manner to form the openings.
For the functional element to be reflective at least on the side of the incident beam, a reflective layer can be applied to this side of the element, or this side of the element can be polished, especially by electrochemical means.
In regard to the arrangement of the openings in the plane of the functional element, the openings can be designed as an evenly distributed point pattern. Any conceivable point distribution can be used such as a rectangular or obliqueangled pattern, or a pattern that optimally exploits the surface of the functional element (e.g. the form of a packed dense sphere). In one embodiment variation, the point pattern can have an uneven distribution in at least one spatial direction so that the beam splitter can be segmented into areas with different divided beam ratios. Alternately, this type of segmentation can also obtained by creating areas with different cross-sections of the openings.
When a beam splitter according to the invention is used in an optical measuring setup, it is preferably rotated or tilted in reference to the optical axis formed by the incident beam so that the reflected beam does not coincide or collide with the incident beam. To effectively prevent a divided beam ratio that depends on the respective angle of rotation, the functional element can also be designed so that its thickness at the openings preferably yields a ratio much smaller than 1 in relation to the cross-section of the opening. The larger the ratio, the lower the yield of the transmitted beam independent of the rotational angle of the beam splitter since the yield can basically only be influenced by additional reflection arising from the edges of the openings.
The openings are preferably square or round. The three-dimensional design of the openings primarily depends on the respective manufacturing procedure, i.e., the design is basically square following the crystal axis when semiconductor material is chemically etched, or round when a metal film is micro-drilled.
Given the relative thinness of the functional element at the openings, the edge of the functional element can be held by a support to increase the overall mechanical stability of the functional element. The support element is preferably designed as a material bead (thickening) of the starting material, i.e., formed by anisotropic etching using a photolithographic mask on a semiconductor material. When a metal film is the starting material, the support element can be created by not thinning the metal film at the edge, i.e., by using a suitable mask when the metal film is chemically thinned.
Finally, when a semiconductor material is used, the functional element can be advantageously manufactured by placing numerous functional elements on a wafer. This allows the beam splitter according to the invention to be mass-produced similar to semiconductor chips in the field of microelectronics.